1. Field of the Invention
This invention relates generally to electrochromic displays, and in particular relates to the use of polyisothianaphthene (PITN) polymer membranes in electrochromic displays.
2. Description of the Prior Art
With the latest remarkable developments in lightening, thinning or miniaturization of electric and electronic instruments, not only lightening, thinning or miniaturization of various conductive materials used therein but also new development of these material per se have been desired.
Various sulfur-containing heterocyclic polymers are known including polymers from thiophene, U.S. Pat. Nos. 2,552,796 and 2,658,902; polymers from dibenzothiophene, U.S. Pat. No. 3,585,163; polymers from vinyl bithiophene, U.S. Pat. No. 3,615,384; polymers from various substituted thiophenes, U.S. Pat. No. 3,725,362; polymers from 2-bromo-8-hydroxy-5,5-dioxodibenzothiophene, U.S. Pat. No. 3,775,368; and polymers from tetrathiapentalene, U.S. Pat. No. 4,111,857.
Within the rapidly expanding field of polymeric conductors ("Proceedings of the International Conference on the Physics and Chemistry of Polymeric Conductors", J. Physigue. Colloque., (1983), C-3), the poly(heterocycles) have received attention because they are easily prepared in film form and are considerably more stable to atmospheric exposure than polyacetylene or polyphenylene. For use in stabilizing a semiconductor surface, see R. Noufi et al., J. Amer. Chem. Soc., (1981), Vol. 183, 184 and references therein. A further extension of this work is our recent entry into the study of polythiophene.
Extensive investigations on new conductive polymers have been conducted. For example, polyacetylenes are under investigation for possible availability as electrode materials of secondary batteries since they show conductivities as high as 10.sup.2 to 10.sup.3 S/cm when doped with iodine or arsenic pentafluoride (cf. Synthetic Metals, Vol. 1, No. 2, 101 (1979/1980)). These polymers also display excellent charge-discharge characteristics. Use of polyacetylenes in solar batteries is also under investigation because of the polymers' light absorption characteristics which are close to those of sunlight. However, the polyacetylenes are disadvantageous in that they are per se susceptible to oxidation and doped polyacetylenes are extremely sensitive to humidity.
Polythiophenes have been studied not only as conductive materials and as battery electrode materials, but also as electrochromic materials making use of color changes in a doped state. For example, A. M. Druy, et al reported that 2,2'-bithienyl may be electrochemically polymerized to form a polymer having a color which reversibly varies from blue in an oxidized state to red in a reduced state, thus a potentially useful electrochromic material [cf. Journal de Physioue, Vol. 44, No. 6, C3-595 (1983)]. However, polythiophenes, like polyacetylenes, are generally sensitive compounds.
In light of the above-described problems, the present inventors have conducted extensive investigations into conductive polymers and uses thereof, and, as a result, have found that a polymer having an isothianaphthene structure (Formulae A-D) is a very stable compound which is capable of continuously and reversibly varying its color in the course of oxidation or reduction. These novel polymers, which display conductivities higher than about 10.sup.-2 S/cm upon doping with general dopants, are described in U.S. application Ser. No. 736,984 filed May 22, 1985, now issued as U.S. Pat. No. 4,640,748 (the parent hereto), the disclosure of which is hereby incorporated by reference in its entirety. Preferred methods of making such polymers are set forth in that patent disclosure as well.
While not wishing to be bound by any particular theory, we believe that polyisothianaphthene, a polymer of a "nonclassical" thiophene, exhibits greater stability and perhaps conductivity than polythiophene because the resonance contributors represented by Formula C and Formula D provide for stabilization of the structure as well as delocalization along the backbone. ##STR1##
We have found that the most desirable approach to polyisothianaphthene is through the electrochemical coupling of isothianaphthene. The isothianaphthene monomer may be prepared according to the methods set forth in J. A. Gadysz et al., Tetrahedron, (1979) Vol. 35, 2239; M. P. Cava et al., J. Amer. Chem. Soc., (1959), Vol 81, 4266; M. P. Cava et al., J. Org. Chem., (1971) Vol. 36, 3932. See, e.g., U.S. application Ser. No. 736,984, cited supra and previously incorporated by reference.
Because polyisothianaphthenes are very stable and exhibit extremely rapid p-type electrochemical doping characteristics with an associated high contrast color change, many applications of the polymers are clearly feasible. One application--use in electrochromic displays--is made possible by virtue of the fact that polyisothianaphthene is a transparent, as well as conductive, polymer. After doping, a thin film of PITN has a very low optical density in the visible portion of the spectrum, as will be demonstrated herein. Other applications which make use of the opto-electrochemical properties of the novel polymers are encompassed by the present invention as well, e.g., use of PITN on or as an electrode in a battery or electrochemical cell, use in a solar energy conversion device, and general use as (or in) electrochromic material.
In a number of ways, electrochromic displays represent an improvement over liquid crystal devices which have recently been developed as "low-energy" display devices with, potentially, a wide range of applications. The display in liquid crystal devices is dependent on visual angle and the contrast and resolution are typically relatively poor. No memory function is provided, nor can the display be provided over a large surface area. In order to eliminate these disadvantages, studies have been extensively conducted on low-energy electrochromic display (ECD) devices which make use of a material's electrochromic properties, i.e., electrochromic materials have light absorption characteristics which vary with application of voltage or electric current. Electrochromic materials which can be used in ECD devices may be either inorganic or organic. Inorganic materials that are considered usable mainly include oxides of transition metals, but these are limited with respect to developable colors. Transition metal oxides also cause electrochemical elution of the membrane or deterioration of electrodes when protons are used as color-forming ions, although response speeds may be high. Organic materials used in electrochromic displays typically include viologen dyes, phthalocyanine complexes, etc. However, the viologen dyes are disadvantageous in that repeated use thereof results in precipitation of insoluble substances, and the phthalocyanine complexes have an as-yet unsolved problem with adhesiveness between a vacuum-evaporated membrane and a base plate.
Other electrochromic materials which have recently been proposed include: polyanilines as disclosed in A. F. Diaz, et al., Journal of Electro-Analytical Chemistry, Vol. 111, 111 (1980) or Yonemaya et al., ibid, Vol. 161, 419 (1984); polypyrroles as disclosed in A. F. Diaz et al., ibid, Vol. 101 (1983) and polythiophenes as disclosed in M. A. Druy, et al., Journal de Physique, Vol. 44, June, page C3-595 (1983) or Kaneto et al., Japan Journal of Applied Physics, Vol. 23, No. 7, page L412 (1983). However, none of these materials has been put to practical use. Preferred characteristics for electrochromic materials are rapid response time in electrochromic switching, high contrast and resolution, good color tones and the like. Furthermore, as noted above, an electrochromic material such as PITN which is capable of developing a colorless tone will greatly contribute to the applicability of the device. This is in contrast to the aforementioned heteroconjugated materials which are colored in the course of conversion from an oxidized state to a reduced state.